A mere fan you say? No, that's a tessen or an iron fan, one of the top 8 Weird Asian Martial Arts Weapon at Weird Asia News:
Folding fans with outer spokes made of iron which were designed to look like regular, harmless folding fans or solid clubs shaped to look like a closed fan.
Samurai could take these to places where swords or other overt weapons were not allowed, and some swordsmanship schools included training in the use of the tessen as a weapon.
The tessen was also used for fending off arrows and darts, as a throwing weapon, and as an aid in swimming, like hand-flippers.